Thermoplastic polymer parts find application in many industries of which the automotive industry is one. Such parts may be formed by processes such as injection molding, thermoforming, and blow-molding among others. Whatever the process, if the part is to be painted, the surface must comply to exacting standards as painting may not mask surface defects or may exaggerate them.
Such a surface is known in the automotive industry as a "Class A surface" although strictly speaking a surface is rated Class A only after painting.
A surface sufficiently smooth of contour, uniform of texture, and free from pits, pores, die lines, weld and flow marks, and inclusions to permit painting a Class A surface can be prepared by operations such as sanding and polishing subsequent to molding. Economics, however, dictates molding a surface of sufficient quality that it may be painted without secondary operations except the removal of flash.
Large parts, such as panels, are difficult to form with a Class A finish or any other surface replicating the negative image on a mold face by injection molding particularly since the structural requirements of large parts are best satisfied by resins which exhibit high melt viscosity. These parts are difficult or impossible to form by that method due to the combination of high viscosity and the need for long thin injection passages to avoid weld lines. This in turn leads to high injection pressures, enormous mold clamping forces and massive molds. Viscosity modifiers help to a degree but introduce other problems.
These large parts then are either thermoformed (pressed out of a preform, a heated sheet) by the vacuum/pressure or the matched mold method or blow-molded (expanded from a preform, usually a tubular parison) by the injection blow molding method in which the hot parison is formed just prior to expansion. These two procedures also have problems. In these processes where, as is customary, a hot sheet is pressed, or a hot parison is expanded into a mold, air may be trapped between the soft surace and the mold, causing an imperfection. If the mold is hot, a good part can be made if the part is small. If the part is large, the mold surface is not replicated and/or surfaces may be thermally degraded where the air pockets prevent cooling. Installing strategically located vents may remove these air pockets, but the vents themselves are prone to mark the final part. Using a cold mold tends to yield parts that have the aforementioned surface defects particularly weld lines, die lines, pits and inclusions.
It, thus, is desirable to form thermoplastic parts which have at least one critical surface that replicates the mold surface without imperfections and are not subject to such defects as voids, pores, die lines, weld lines, inclusions and the like.